The present invention relates to a polishing pad for chemical mechanical polishing.
In manufacturing a semiconductor device, a step of forming an conductive film over the surface of a wafer is followed by a step of forming a wiring layer by photolithography, etching or the like and a step of forming an inter-layer insulation film over the wiring layer. These steps produce non-uniformity of the wafer surface. As fineness of wiring is increased and multi-layered wiring is used in recent years for higher density semiconductor integrated circuits, a technique for planarizing a non-uniform wafer surface has been important.
Methods for planarizing a non-uniform wafer surface include what is known as a chemical mechanical polishing (CMP) method. In the CMP method, slurry in which abrasive grains are dispersed in a liquid is used as a polishing solution, the surface of the wafer to be polished is pressed against the polishing surface of a polishing pad and polished.
A polisher for use by the CMP method is provided with, for instance a polishing table 2 for supporting a polishing pad 1, a supporting base 6 for supporting an object (wafer) 5 of polishing and a feed mechanism 10 for the polishing solution as illustrated in FIG. 1. The polishing pad 1 is fixed to the polishing table 2 with a double-sided adhesive tape or otherwise. The polishing table 2 and the supporting base 6 are so arranged that the polishing pad 1 and the object 5 be opposite each other, and provided with rotation axes 8 and 9, respectively. On the supporting base 6 side, there is provided a pressing mechanism for pressing the object 5 against the polishing pad 1.
In polishing a wafer surface by the CMP method, it is required to detect, without having to interrupt the progress of polishing, the end point of polishing (the point of time at which the surface structure and the insulating layer thickness of the wafer achieve their respectively desired states). As a way of detecting this end point of polishing, the wafer surface can be irradiated with a laser beam through a polishing pad and the beam reflected from the wafer can be monitored.
The reflected beam from the wafer having an insulating film on the surface contains an interference light resulting from interference between a first reflected light reflected by an insulating film face present on the wafer surface and a second reflected light reflected by a boundary face between the insulating film and a silicon substrate. This interference light has an intensity matching the phasic relationship between the first reflected light and the second reflected light, and this phasic relationship represents the thickness of the insulating film over the silicon substrate. Therefore, the end point of polishing can be detected by monitoring the reflected light from the wafer and analyzing the interference light.
This method for detecting the end point of polishing is described in, for instance, in Japanese Patent Laid-Open No. 9-7985 (U.S. Pat. No. 5,964,643), WO 99/64205 (internationally disclosed after the priority date of the present application), Japanese Patent Laid-Open No. 10-83977 (U.S. Pat. No. 5,893,796), U.S. Pat. No. 6,045,439 and National Publication of International Patent Application No. 11-512977 (U.S. Pat. No. 5,605,760).
Detection of the end point of polishing by this method requires light transmission areas in the polishing pad. A laser beam is brought to incidence on the wafer surface through the light transmission areas of the polishing pad, lights having passed these light transmission areas, out of the reflected lights from the wafer are directed toward a detector.
The references cited above also describe how these light transmission areas are provided. For instance, a through hole is bored in part of the polishing pad, a hole penetrating the table in its thickness direction is bored continuously from the through hole in the pad, and window members, such as transparent sheets, plugs or the like are fitted to these continuous holes. As these window members, members of a uniform structure consisting of quartz, polyurethane or the like (members having no intentionally designed distribution of refractive index) are used.
However, these methods according to the prior art have need for some improvement in the point of view of the efficiency of bringing reflected lights from the wafer to incidence on the photo detector.
As polishing of a wafer cannot completely eliminate non-uniformity on the wafer surface even if the polishing is done to the end point, the reflected lights from the wafer are scattered. If the face of the window member toward the polishing face is more depressed than the polishing face itself, the polishing solution having accumulated in this more depressed part further scatters the reflected lights from the wafer. If the face of the window member toward the polishing face is made level with the polishing face, the face of the window member toward the polishing face may also be polished depending on its material, resulting in further scattering of the reflected lights from the wafer by the face to be polished.
Therefore, even if a light normal to the polishing face is brought to incidence through the window member, the reflected lights from the wafer will not be aligned to the direction normal to the polishing face. As a result, when these reflected lights enter the window member of a uniform structure, part of these reflected lights will be absorbed by, for instance, the inner face of the through hole in the table and fail to reach the detector.
It is conceivable to expand the light transmission area to bring the reflected lights from the wafer to incidence on the photo detector efficiently, but an expansion of the light transmission area would reduce the polishing face of the polishing pad correspondingly. Thus, it is not preferable to expand the light transmission area because it would adversely affect the uniformity of polishing.
To add, WO 99/64205 describes an arrangement in which a laser beam is brought to incidence and reflected lights are received by an optical fiber, one end of this optical fiber is inserted into a through hole bored in the polishing pad, and the other end is connected to a light receiver for detecting the end point of polishing. Thus, in this example, no window member is fitted in the light transmission area of the polishing pad.
An object of the present invention is to make a transparent window member (provided in the light transmission area of a polishing pad for detecting the end point of polishing by a CMP method) a composition which enables reflected lights from a wafer to be efficiently brought to incidence on a photo detector, even if the size of the transparent window member is small.
In order to solve the problems noted above, the present invention provides a polishing pad for chemical mechanical polishing having a polishing area and a light transmission area consisting of a transparent window member within a pad surface, wherein the window member has areas of a high refractive index and areas of a low refractive index in its window face, and each of the areas is alternately arranged in stripes in a cross section normal to the window face.
When a light is brought to incidence on the light transmission area of the polishing pad from one window face, the light travels in the thickness direction of the polishing pad mainly in the areas having a high refractive index while being reflected by the boundary between the areas having a high refractive index and the areas having a low refractive index, and is emitted from the other face. Thus, even if the light coming incident on this light transmission area is not uniform in direction, the light is transmitted substantially in the lengthwise direction of the aforementioned stripes within the light transmission area.
Therefore, this light transmission area, where the incident light is not uniform in direction, can make the degree of diffusion of the light emitted from the light transmission area lower than does a light transmission area of a window member of a uniform structure. Accordingly, this polishing pad can bring a reflected light (light not uniform in direction) from the object of polishing for detecting the end point of polishing to incidence on the photo detector more efficiently than a polishing pad provided with a light transmission area of a window member of a uniform structure.
It is preferable for the polishing pad according to the invention that the arrangement of the areas of a high refractive index and the areas of a low refractive index constituting the window member be a Fresnel zone plate arrangement in which the areas of a high refractive index are matched with the bright area of a Fresnel zone plate and the areas of a low refractive index are matched with the dark area of the Fresnel zone plate.
As the arrangement of the areas of a high refractive index and the areas of a low refractive index constituting the window member of this polishing pad is the Fresnel zone plate arrangement, the window member has a light condensing effect similar to that of a Fresnel zone plate in addition to the aforementioned effect of transmitting the incident light in the light transmission area substantially in the lengthwise direction of the stripes (optical wave-guiding effect). For this reason, when a light is brought to incidence on the light transmission area of this polishing pad from one face, the light emitted from the other face is condensed. Thus, even if the light coming incident on this light transmission area is not uniform in direction, the light emitted from this light transmission area is focused.
Therefore, this polishing pad enables a reflected light(light not uniform in direction) from the object of polishing for detecting the end point of polishing to be emitted from the light transmission area as a focused light. As a result, this light transmission area can bring a reflected light from the object of polishing to incidence on the photo detector more efficiently than a light transmission area of a window member of a uniform structure. Furthermore, it can bring a reflected light from the object of polishing on the photo detector more efficiently than a light transmission area of a window member covered by the invention but having no Fresnel zone plate arrangement.
A Fresnel zone plate, as shown in FIG. 2, is a pattern consisting of a plurality of concentric circles, a first area Z1 corresponding to the inside of a first circle C1 of this pattern, a second area Z2 corresponding to the space between the first circle C1 and a second circle C2, a third area Z3 corresponding to the space between the second circle C2 and a third circle C3 and so forth, the first circle C1, the second circle C2, the third circle C3 and so forth being counted from the center outward, are alternately bright areas (light transmission are as) and dark are as (light intercepting areas). The relationships among the circles C1, C2, C3 and so on are such that the radius Rn of an n-th circle is proportional to the square root of (2nxe2x88x921). This causes diffracted lights from the bright areas interfere with each other in the same phase to have a light condensing effect.
The focal length of the Fresnel zone plate differs with the wavelength, and the relationship among the radius Rn, the focal length P of each concentric circle and the wavelength xcex can be represented by Equation (1) below. Generally, a Fresnel zone plate having a desired focal length is designed by substituting the wavelength xcex of the incident light and the desired focal length P into this Equation (1) and thereby deriving the radius Rn of each concentric circle.
Rn={square root over ( )}(xcexxc2x7P(2nxe2x88x921)/2)xe2x80x83xe2x80x83(1)
This Equation (1) is mentioned in, for instance, Keigo Iizuka, Hikari Kogaku (Optical Engineering) (expanded and revised new edition), 1983, Kyoritsu Shuppan Kabushiki Kaisha, p. 68.
A window member of the Fresnel zone plate arrangement according to the invention can also be designed to have a desired focal length by deriving the radius Rn of each concentric circle from this Equation (1). Further in this window member, the first area Z1 of the Fresnel zone plate may be either an area of a high refractive index or an area of a low refractive index, but it should preferably be an area of a high refractive index. By having an area of a high refractive index as the first area Z1, more areas of a high refractive index can be arranged in the window member than where an area of a low refractive index is arranged as the first area Z1, resulting in a higher optical wave-guiding effect.
It is preferable for the window member of the polishing pad according to the invention to have a refractive index difference, represented by xe2x80x9c(n1xe2x88x92n2)/n1xe2x80x9d, of not less than 0.5% but not more than 10%, wherein n1 is the refractive index of the areas of a high refractive index and n2 is the refractive index of the areas of a low refractive index, even more preferably not less than 1% but not more than 10%. Too narrow a refractive index difference would reduce the optical wave-guiding effect.
Where the refractive index difference is greater than 10%, there arise significant differences between the areas of a high refractive index and the areas of a low refractive index in the physical properties of materials, including specific gravity and hardness, though the optical wave-guiding performance does not drop, and accordingly it is made difficult to form the window member to constitute the light transmission area. Also, where the refractive index (n1) of the areas of a high refractive index is too high, the proportion of lights reflected by the surface of the light transmission area increases, which is not desirable.
It is preferable for the polishing pad according to the invention that the proportion of the areas of a high refractive index in the window member be not less than 15% but not more than 90% in terms of their square measure in the window face. If the proportion is less than 15% or more than 90%, the optical wave-guiding effect may become insufficient. Taking account of the relative greatness of the optical wave-guiding effect and the relative ease of fabricating the window member constituting the light transmission area, the preferable range of the proportion not less than 20% but not more than 80%, and an even more preferable range is not less than 50% but not more than 80%.
It is preferable for the polishing pad according to the invention that the areas of a high refractive index in the window member be formed in a columnar shape whose axial direction is normal to the window face, and the diameter of this column be not less than 50 xcexcm but not more than 2000 xcexcm. This polishing pad would provide an especially high optical wave-guiding effect.
If the diameter is less than 50 xcexcm, optical diffraction on the boundary between the areas of a high refractive index and the areas of a low refractive index will become significant, resulting in a reduced optical wave-guiding effect. Also, if the diameter is more than 2000 xcexcm, the optical wave-guiding effect will drop. It is preferable for the diameter be not less than 50 xcexcm but not more than 500 xcexcm, even more preferably not less than 75 xcexcm but not more than 200 xcexcm.
Where the polishing pad according to the invention has the Fresnel zone plate arrangement of the areas of a high refractive index and the areas of a low refractive index, there may either be only one such Fresnel zone plate arrangement or a plurality of such arrangements. Where there are a plurality of such arrangements, it is preferable for the diameter of the outermost ring constituting a bright area of the Fresnel zone plate to be not less than 300 xcexcm but not more than 2000 xcexcm and for the width of this outermost ring to be not less than 10 xcexcm but not more than 200 xcexcm.
If the outer diameter of the outermost ring is less than 300 xcexcm or the width of the outermost ring is less than 10 xcexcm, the impact of diffraction on the refractive index boundary will increase, making it difficult to achieve a light condensing effect similar to that of the Fresnel zone plate. If the outer diameter of the outermost ring is more than 2000 xcexcm or the width of the outermost ring is more than 200 xcexcm, it will also become difficult to achieve a light condensing effect similar to that of the Fresnel zone plate.
A composition in which the window member has only one Fresnel zone plate arrangement would make it possible to reduce the light receiver size, and this would be preferable where a light of a large beam diameter is to irradiate the light transmission area. By contrast, a composition in which the window member has a plurality of Fresnel zone plate arrangements would result in a plurality of light condensing points, and this would provide the advantage of making it possible to receive reflected lights more dependably where reflected lights come incident on only part of the window member face.
It is preferable for the polishing pad according to the invention that the window member constituting the light transmission area consists of cross-linked polymers. Generally, the higher the level of cross linking, the denser the cross-linked polymer. Therefore, it is possible to vary the refractive index of a member consisting of cross-linked polymers by controlling the level of cross linking when polymers are cross-linked. Where the polishing pad according to the invention has a window member consists of cross-linked polymers, the areas of a high refractive index have a higher level of cross linking of polymers than the areas of a low refractive index.
Since cross-linked polymers are chemically stable, a window member consisting of cross-linked polymers is hardly affected by the polishing solution used in the CMP method. Furthermore, if a difference in refractive index is generated by controlling the level of cross linking of polymers, the areas of a high refractive index and the areas of a low refractive index take on a state of firm bonding by chemical bonding. For this reason, a window member consisting of cross-linked polymers can hardly be broken even when subjected to mechanical deformation.
Where the window member of the polishing pad according to the invention is to be formed of cross-linked polymers, photosensitive polymers, for instance, can be used as the cross-linked polymers. By so irradiating the photosensitive polymers with a light that the level of cross linking in the areas of a high refractive index be higher than that in the areas of a low refractive index according to the refractive index distribution on the pad face, a window member having a refractive index distribution can be obtained.
Suitable photosensitive polymers include polyurethane acrylates, epoxy acrylates, polyester acrylates, unsaturated polyesters, rubber acrylates, polyamides, silicon acrylates, alkyd acrylates and cyclized rubbers. Polybutadienes are also preferable because their high resistance to acid and alkali serves to prevent deterioration by the polishing solution used in the CMP method.
Resin mixtures containing one or another of these photosensitive polymers can as well be used. In this case, a desired level of hardness can be given to the photosensitive polymers by adjusting the composition of the resin mixture and the quantities of monomers (acrylates, methacrylates or multifunctional monomers having a vinyl group) to the photosensitive polymers.
It is preferable for the polishing pad according to the invention that the window face of the window member on the polishing face side be in the same plane as the polishing face, and that at least the part of the window member on the polishing face side has no greater hardness than the polishing face, the difference in hardness being no more than 20 in Shore-D hardness index.
This polishing pad, because the window face of the window member on the polishing face side is in the same plane as the polishing face, it is difficult for the polishing solution to stay on the aforementioned window face of the window member. In addition, structures that can prevent the polishing solution from staying on the window face of the window member on the polishing face side include one in which the window face protrudes beyond the polishing face, but this structure involves the problems of impossibility of uniform polishing, difficulty of dressing for maintenance and occurrence of scratch on the face to be polished.
Also, since at least the part of the window member on the polishing face side of this polishing pad is no harder than the polishing face, the window face of the window member on the polishing face side does not protrude beyond the polishing face in the process of polishing. Moreover, as the hardness difference is not more than 20 in Shore-D hardness index, even if the window face of the window member is depressed below the level of the polishing face in the process of polishing, this depression can be kept sufficiently small. A more preferable hardness difference is 10 or less in Shore-D hardness index. Further, at least the part of the window member on the polishing face side of this polishing pad should be hard enough not to be damaged while being polished or dressed.
The invention also provides a light-transmissive sheet having face areas of a high refractive index and areas of a low refractive index in the sheet, and each of the areas is alternately arranged in stripes in a cross section normal to the sheet face.
When a light is brought to incidence on one of the faces of this sheet, the light travels in the thickness direction of the sheet mainly in the areas having a high refractive index while being reflected by the boundary between the areas having a high refractive index and the areas having a low refractive index, and is emitted from the other face. Thus, even if the light coming incident on this light transmission area is not uniform in direction, this light is transmitted substantially
Also, since at least the part of the window member on the polishing face side of this polishing pad is no harder than the polishing face, the window face of the window member on the polishing face side does not protrude beyond the polishing face in the process of polishing. Moreover, as the hardness difference is not more than 20 in Shore-D hardness index, even if the window face of the window member is depressed below the level of the polishing face in the process of polishing, this depression can be kept sufficiently small. A more preferable hardness difference is 10 or less in Shore-D hardness index. Further, at least the part of the window member on the polishing face side of this polishing pad should be hard enough not to be damaged while being polished or dressed.
A light-transmissive sheet having face areas of a high refractive index and areas of a low refractive index in the sheet can also be used as a window member of a polishing pad of the present invention, and each of the areas is alternately arranged in stripes in a cross section normal to the sheet face.
When a light is brought to incidence on one of the faces of this sheet, the light travels in the thickness direction of the sheet mainly in the areas having a high refractive index while being reflected by the boundary between the areas having a high refractive index and the areas having a low refractive index, and is emitted from the other face. Thus, even if the light coming incident on this light transmission area is not uniform in direction, this light is transmitted substantially in the lengthwise direction of the aforementioned stripes within the light transmission area.
A sheet of the above described composition may include a sheet wherein the arrangement of the areas of a high refractive index and the areas of a low refractive index in the sheet face is a Fresnel zone plate arrangement in which the areas of a high refractive index are matched with the bright areas of a Fresnel zone plate and the areas of a low refractive index are matched with the dark areas of the Fresnel zone plate. This sheet, by virtue of the arrangement of the areas of a high refractive index and the areas of a low refractive index, has a light condensing effect similar to that of the Fresnel zone plate in addition to the aforementioned optical wave-guiding effect. Thus, when a light is brought to incidence on one face of this sheet, the light emitted from the other face is condensed.
Therefore, by forming an opening in the light transmission area of a polishing pad for CMP use and arranging any one of the sheets described in the opening, a polishing pad according to the invention can be readily formed.
It is preferable for these sheets to be manufactured by a method whereby the areas of a high refractive index and the areas of a low refractive index are formed by varying the level of cross linking of cross-linked molecules in the sheet face.
A polishing pad of the invention may includes a polishing pad for chemical mechanical polishing having polishing areas and a light transmission area consisting of a transparent window member within a pad surface, wherein the face of the reverse side to the polishing face is fixed to a light-transmissive supporting body, a light-transmissive sheet is arranged in an opening formed in the light transmission area, and the whole surface of this sheet is stuck to the supporting body with a light-transmissive adhesive.
Since in this polishing pad the whole surface of the sheet is stuck to the supporting body with an adhesive, infiltration of the polishing solution into the back face of the sheet (the face arranged on the table side) is more effectively prevented than in a polishing pad wherein only the edge of the sheet is stuck to a supporting body. Furthermore, because of the use of the light-transmissive supporting body and adhesive, a light irradiating from the back side of the sheet can be brought to incidence into the sheet more reliably.
The invention also provides a polishing pad of this kind wherein the sheet has the areas of a high refractive index and the areas of a low refractive index in the sheet face, and each of the areas is alternately arranged in stripes in a cross section normal to the sheet face.
The invention also provides a polisher having light irradiating means for irradiating an object of polishing via the light transmission area of a polishing pad with a laser beam of a single wavelength or a light of a narrow wavelength range having passed a band pass filter; light receiving means for receiving, light having passed the light transmission area, out of the reflected lights from a wafer; and end point detecting means for detecting the end point of polishing according to a light reception signal from the light receiving means, wherein the polishing pad is a polishing pad according to the invention.
The invention also provides an end point of polishing detection method which comprises irradiating a wafer surface with a laser beam of a single wavelength or a light of a narrow wavelength range having passed a band pass filter through the light transmission area of a polishing pad, monitoring reflected lights from the wafer through the same light transmission area, wherein the polishing pad used is a polishing pad according to the invention.